The present invention relates generally to flaw detection in solid materials and, more particularly, to flaw detection employing non-destructive testing techniques.
Non-destructive testing to identify and/or isolate flaws or discontinuities in solid materials have typically employed X-ray and ultrasonic techniques among others. X-ray techniques, although providing relatively complete information about the extent of flaws in solid materials such as metal, require that the metal structure be in a form which permits the X-rays to pass completely therethrough from X-ray source to film. In the case of very thick sections of metal, exposure times and dynamic range interfere with obtaining good data. In addition, X-ray testing is time consuming and costly.
Monostatic pulse-echo ultrasonic testing techniques, wherein transmitting and receiving transducers are generally collocated, have been successfully employed in the detection of flaws in critical objects such as the rotors of turbines and generators. Such use is disclosed in a paper entitled,"Boresonic Inspection of Forged Turbine and Generator Rotors" by W. R. Marklein and R. E. Warnow presented to the American Society of Mechanical Engineers at its annual meeting in New York, Nov. 29 to Dec. 4, 1964. In this report, a pulse-type ultrasonic transducer launches pulses of ultrasonic vibration into a forging from within an axial bore. Cracks, tears or non-metallic inclusions in the forging produce discontinuities which reflect the ultrasonic energy to the receiving transducer. The received reflected ultrasonic energy cannot be directly correlated with the actual size of the flaw. However, in the absence of better techniques for testing large forgings in critical applications, the above ultrasonic methods have been developed into a commercial operation and routinely applied in the non-destructive testing of large turbine and generator forgings.
Flaws fall into two general classifications, namely point flaws and extended flaws. Point flaws, which may be due to porosity or small non-metallic inclusions, have dimensions which are too small to provide more than a single indication to an ultrasonic sensor. Extended flaws have dimensions large enough so that a plurality of resolvable indications may be received.
One of the reasons that the ultrasonic return from an extended flaw is poorly correlated with the size of the flaw is that the reflection of ultrasonic energy from a flaw is fairly critically dependent on the angular relationships of the direction of propagation of the ultrasonic energy and the orientation of the flaw. Maximum ultrasonic return is achieved when the flaw is disposed normal to the direction of propagation of the ultrasonic energy. Thus, a very large return may be obtained from a relatively small crack fortuitously disposed normal to the direction of propagation of the ultrasonic energy whereas a relatively small return may be received from a relatively large and serious flaw disposed at an oblique angle to the direction of propagation of the ultrasonic energy.
The term ultrasonic is generally taken to mean mechanical vibration at frequencies higher than the audible range. That is, higher than about 20 KHz. For non-destructive testing in metals, ultrasonic frequencies in the range of from about 1 to about 10 MHz are customarily employed.